Large power grid interconnection is an essential trend in the development of power grids all over the world, which however may lead to the problem of low frequency power oscillation at oscillation frequencies from 0.1 Hz to 2.0 Hz in general. If the oscillation mode has a weak damping or even negative damping, it may cause grid instability and endanger grid security.
In the 1960s, low frequency power oscillation happened in North America power systems. Since 1980s, with the increased percentage of large generator units with fast excitation systems and large-scale power grid interconnection, low frequency oscillation phenomena occurred in China's power grids, for example, in Hunan power systems in 1983, in the tie-lines of the Guangdong-Hong Kong interconnection system in 1984 and 1985, in the interconnection system of the China Southern power grid system in 1994, in the Ertan power transmission system of the Sichuan-Chongqing power grid in 1998 and 2000, in China Southern-Hong Kong AC/DC power transmission system in 2003, etc. How to suppress low frequency oscillation of interconnected system is an important issue to be solved for large power grid interconnection.
Currently, the following measures are practically adopted in projects to suppress low frequency oscillation in power systems:
1. Adding power system stabilizers in generator excitation systems to suppress power system low frequency oscillation, which is a measure commonly adopted in power systems of China at present.
2. Applying HVDC power regulation to suppress low frequency oscillation in power systems. For HVDC transmission system or AC/DC hybrid systems, a low frequency power oscillation damping module may be added in a HVDC control system to increase system damping for the potential low frequency oscillation mode.
3. Adopting FACTS devices, for example, thyristor controlled series compensator (TCSC), static var compensator (SVC) may be used to suppress low frequency oscillation in power systems. For example, TCSC is installed at the tied line between the Brazil's northern and southern power grids, which suppress low frequency oscillation by TCSC impedance control.
However, the above measures are limited. Adding power system stabilizers in generator excitation systems to suppress power system low frequency oscillation is limited in that: (1) because a power system has a large amount of generators widely distributed, providing a power system stabilizer for each generator needs a lot of works; (2) in practical operation, the power system stabilizers of some power plants may out of operation due to management or technical reasons, as a result, damping effects may be decreased; (3) the parameter designs of power system stabilizers must be coordinated with each other and adapted to different configuration of power systems If the configuration of the power system or operation mode of generators has been changed significantly, the effects of the power system stabilizer with preset designed parameters may be limited for suppressing the low frequency oscillation.
Applying HVDC regulation function or adopting FACTS devices such as TCSC or SVC to damp low frequency oscillation, these devices must be installed in the power systems. Meanwhile, TCSC and SVC are devices based on impedance adjustment or reactive power adjustment, which are indirect measures to suppress active power oscillation, and are affected by system configuration apparently.
In summary, in order to adapt to the new trend of large power grid interconnection, innovative solutions to dynamically recognize the low frequency oscillation mode of power system, design, measures and devices of the controller are desired.